It is known that combination electrodes, which consist of an internally cooled electrode holder with an attached active part of carbon material, have been employed in electric-arc furnace operations for some time. The electrode holder of metal or alloys serves not only as mechanical fastener of the active part but also acts as current supply. DE-AS No. 24 30 817, the German document laid open to public inspection, describes e.g. an electrode for electric-arc furnaces which has an upper, internally cooled metal electrode holder that remains in the clamping jaw zone during operation. Electrode sections of graphite are screwed to its lower part. The current is supplied via clamping jaws enclosing the metal sheath area of the electrode holder. As the current supplying clamping jaws rest directly on the sheath area of the electrode holder, the electrode holder may be mechanically damaged. With cooling water pipes on the inside of sheath area, this danger is especially critical, for their damage may lead to a leak and, consequently, to the escape of water into the hot molten metal.
DE-AS No. 27 39 483 already describes electrodes for electric-arc furnaces of the type mentioned where the metal shaft of the electrode support holding the active part and a clamp inserted from the outside are connected by means of a metal-metal contact. With this type of design, the sheath area of the electrode holder constitutes the outer limit for the backflow of the cooling agent. Here, too, mechanical damage may as a result of the clamping force exerted by the clamping jaws supplying the current. As the current supply is based on a metal-metal contact, the metallic sheath area of the electrode holder is not protected against mechanical or electrical impairments such as arching, which is the reason why the electrode holder cannot be inserted into the interior of the electric-arc furnace. Depending on the dimensions of the electric-arc furnace, it is therefore necessary to attach relatively long active parts to the electrode holder, which leads to an increase in the consumption of active material. Furthermore, the manoeuvrability of the electrode within the electric-arc furnace is rather limited.
In its European patent application 80 106 583.0 the applicant already suggested to equip the outer sheath area of the electrode holder with intercalations which may be fastened by means of pocket mountings. Although such a contact zone at the upper end of the metal shaft with a length of approximately 0.2 m to 0.5 m has its advantages, it does not in all cases produce the flexibility when employing the electrode
All these electrode holders have one disadvantage in common, that is, as the tip (the consumable active part) is consumed, the electrode holder has to be lowered to meet the positioning requirements concerning bath level respective scrap distribution.
For conventional electrodes, consisting of a column of e.g. graphite sections, which are screwed one by one on the top of the column as the lower parts are consumed, the range of control which had to be covered by the positioning means was restricted to the actual distance, over which the lower tip end had to be moved in order to be adjusted to the scrap or the bath level. The consumption of the electrode was compensated by feeding the endless electrode from the top and more or less continuously lowering the entire column. With the employment of combined electrodes consisting of a water cooled permanent upper section, the consumption of the tip has to be compensated by axial movement of the permanent section, as far as permitted by the range of the existing positioning means of the established arc furnaces. Since there must always be provided a certain range of axial movement for regulation purposes, only the relatively small difference between the entire range of the positioning means and the necessary control range is left to compensate the consumption of the active parts. When a length of the tip corresponding to that difference is consumed, a new section of graphite or the like has to be screwed onto the lower end of the electrode holder, which constitutes the permanent section.